This invention relates to the art of nuclear reactors and has particular relationship to the upper-internals structure of nuclear reactors. A nuclear reactor includes a pressure vessel into which a heat-transfer fluid, typically liquid sodium for fast breeder reactors, or pressurized or boiling water for more conventional commercial reactors, is pumped under pressure. The fluid flows through the core and is heated; the hot fluid emerges from the vessel and the heat flows via mechanically separated primary and secondary loops to electrical-power generating equipment. Within the vessel there is supporting structure for the core components. Typically, for a liquid-metal-cooled fast breeder nuclear reactor which generates more fissile fule than it burns up, these components include fuel-rod bundles or assemblies, control-rod assemblies, blanket fertile-material or fertile-rod assemblies and removable radial shielding assemblies. The expression "core assemblies" or "core component assemblies" or the word "assembly", when used in this application with reference to components of the core, means one or more types of these assemblies. The core-support structure serves the purposes of locating, supporting, distributing coolant to, and providing axial and radial restraint for, these assemblies.
The core component assemblies, which in the illustrated embodiment include fuel assemblies, both fissile and fertile fuel containing types, control-rod assemblies and shielding assemblies, which form the core of a liquid metal cooled fast breeder nuclear reactor, are separately supported in inlet-support modules or modular units. Each inlet-support modular unit is removably mounted, held only by gravity, in liners in the lower core-support structure with fluid seals interposed between the aligned fluid inlet openings in the module and liner and the upper and lower parts of the module and liner. Each module directs flow of the heat-transfer or coolant fluid to a plurality (typically 7) of reactor component assemblies which are removably mounted, held only by gravity, in receptacles of the corresponding modular unit. Below the seal each module is subjected to low pressure which balances the low pressure in the region where the fluid emerges from the core components. The low pressure in the volume below the module lower seal is generated and maintained by venting this volume to the low pressure regions of the vessel of the reactor. Gravity is adequate to hold the modules in the liner.
Typically, this invention applies to a 975 Megawatts -thermal, 400 Mwt-electrical (Mwt.) liquid-metal cooled fast breeder reactor which has 198 hexagonal-core fuel assemblies surrounded by 150 radial blanket assemblies and 324 radial shield assemblies. In this typical reactor the assemblies are received in 61 inlet modules each having 7 receptacles. The velocity of the heat-transfer or cooling fluid, which is sodium, and its distribution varies with the character of the component or assembly which it cools. The velociyt is about 30 feet per second in non-replaceable components while in replaceable components it may be as high as 50 feet per second at the inlet-lower-temperatures end and 40 feet per second at the outlet-higher-temperature end. In the fuel rod bundles it is 25 feet per second. Eighty percent of the fluid is allocated to the core, 12% to the radial blanket, 1.6% to control assemblies, and the remainder to shielding, bypass and leakage.
Typically a reactor of the type to which to which this invention relates, for example, a sodium-cooled breeder reactor, operates at a bulk coolant temperature differential of 300F.degree. or greater between the core inlet and core outlet. This temperature gradient is not uniform across the core; it fluctuates widely and has peaks in temperature throughout the core caused by core geometry, fuel "Burnup," and deliberate variations in fuel enrichment. Localized temperature variations may also occur by reason of local anomalies in the core such as control assemblies. Also typically, a sodium-cooled breeder reactor undergoes rapid and severe changes in the core outlet temperature because of rapid changes in power load-level during postulated `upset` events such as reactor trips, rapid unloading, etc.
The structure within the reactor vessel above the core, variously called instrument trees, upper core support structures, or upper-internals structure, or upper internals as it is called in this application, provide primary or secondary `holddown` of the reactor core for the contingency that the gravity holddown fails during emergencies such as scram, and support control-rod drivelines and instrumentation. These upper internals are exposed to the core effluent flow, thermal gradients, thermal transient conditions and periodic "stripping" of hot and cold coolant streams. The word "stripping" means the overlap in temperature which occurs between adjacent parts of a reactor, for example adjacent core-component assemblies, which operate at widely different temperatures. The resulting thermal stress and thermal fatigue may reduce the design lifetime of upper-internals structures, which are normally designed for a lifetime equal to that of the reactor itself.
To assure a reasonable or long lifetime for a reactor, the core-outlet liquid-metal flow streams are mixed as they are delivered at the core outlet. This mixing reduces thermal gradients between flow streams at widely different temperatures and isolates the remaining structure of the upper internals from direct impingement by the flow streams, reducing the rate of change for thermal transient events. The mixing is effected by outlet modules, each outlet module serving a plurality of core-component assemblies. These outlet modules collect effluent coolant from the core assemblies and duct it through the above core structure to the reactor outlet plenum. Each outlet module includes a support or `holddown` grid, a flow collector, a chimney, and thermal liners or stubs isolating each chimney from the other upper internals. The support grid is designed to avoid direct impingement of core effluent streams on neighboring parts of the upper internals and it limits the axial travel of the core assemblies below it, thus serving as "holddown" grid.
Core effluent is ducted from the flow collector of each outlet module through the upper internals by the chimneys. Each chimney and its thermal liners protect the upper core support structure from high cycle thermal transients. Flow mixing within the collector and chimney mix hot and cold streams entering the module, providing more even radial gradients between chimneys. The thermal isolation between chimney and `structure` reduces the severity in rate of change for thermal transients due to core power-level changes. It has been found that the mixing of high and low temperature jets of the liquid from the core starts immediately above the core and continues for some distance downstream towards the outlet plenum. Temperatures in these flow streams differ substantially and the mixing of these streams near the inner portion of the outlet modules results in a number of thermal stripping transients. The material selected for the modules must therefore have an endurance limit stress in excess of the maximum anticipated stress amplitude produced by fluid mixing. The part of the outlet module assembly which is subjected to these sharp temperature fluctuations is fabricated from alloys with superior cyclic thermal stress characteristics, while the remainder of the structure is made of relatively inexpensive material.
Typically the part of the assembly which is subject to sharp temperature variations is fabricated from the refractory corrosion-resistant nickel-chromium-iron alloy,, INCONEL-718, and the other parts are fabricated from AISI-304 or 316 stainless steel. INCONEL-718 has the following typical composition:
______________________________________ Nickel 50.00 - 55.00 Chromium 17.00 - 21.00 Columbium (plus Tantalum) 4.75 - 5.50 Molybdenum 2.80 - 3.30 Titanium 0.65 - 1.15 Aluminum 0.20 - 0.80 Cobalt 1.00 Max. Carbon 0.08 Max. Manganese 0.35 Max. Silicon 0.35 Max. Phosphorus 0.015 Max. Sulfur 0.015 Max. Boron 0.006 Max. Copper 0.30 Max. Iron Balance ______________________________________
The 304 stainless steel has the following composition:
______________________________________ Carbon 0.08% Max. Manganese 2.00% Max. Phosphorus 0.040% Max. Sulphur 0.030% Max. Silicon 1.00% Max. Nickel 8.00 - 11.00% Chromium 18.00 - 20.00% Iron Balance ______________________________________
The 316 stainless steel has the following composition:
______________________________________ Carbon 0.08% Max. Manganese 2.00% Max. Phosphorus 0.040% Max. Sulphur 0.030% Max. Silicon 1.00% Max. Nickel 10.00 - 14.00 Chromium 16.00 - 18.00 Molybdenum 2.00 - 3.00 Iron Balance ______________________________________
The cobalt in these alloys and the cobalt and tantalum in the 718 are restricted for use within a reactor vessel. The cobalt and/or tantalum limit is a function of the neutron flux at the location of the material, surface area exposed to primary coolant, velocity of coolant past the exposed area, and the residence time of the material within the reactor vessel. The 718 is not weld compatible with either stainless steel.
Even with the chimneys localized temperature variations occur. Sodium streams, exiting from the chimneys at significantly differing temperatures, mix in the outlet plenum imposing fluctuating temperatures on the surface material of the upper internals. During the scram transient, the section of the upper internals immersed in the sodium or other liquid pool is subjected to a very rapid drop in surface temperature because the control rods are fully inserted in the core. Jet impingement forces from the core outlet flow, and upper plenum cross flow forces are both unsteady, and tend to produce flow induced vibration of the upper internals structure. It has been found that this structure must have adequate structural stiffness. In providing the required stiffness the problem is confronted that only structural configurations which will perform satisfactorily in an ill defined thermal environment can be used.
It has been proposed that the necessary stiffness be achieved by providing a cross-braced frame configuration between the columns in the area below the head plate and the tops of the chimneys. This proposal has proved unsatisfactory because of its sensitivity to thermal inertia matching of the structural members. The expression "thermal inertia" means the facility of a structure to resist temperature change produced by thermal gradients. Structures having higher moments of inertia transmit thermal strains more readily than structures having lower moments of inertia. For example, a corrugated plate transmits thermal strains more readily than an equivalent flat plate. It is essential that any structural member take up the strain arising from the stresses by its flexibility rather than transmitting the strain.
Effective utilization of the reactor vessel outlet plenum mixing volume is essential for mitigation of the transients experienced by the reactor vessel and all the hot leg components. The natural flow characteristics in the outlet plenum assures this to an extent but difficulties are encountered in the case of a scram transient. Stratification of the cool core effluent following a scram have led to concerns that adequate outlet plenum mixing may not occur unless forced. The upper-internals structure outlet-module chimneys provide a means for forcing the required mixing by ensuring that a major portion of the core effluent exits into the plenum at a high elevation. However, a serious problem is presented in fabrication of the complete structure including the chimneys because the highly refractory, high corrosive resistant nickel-chromium alloy of which the chimneys are composed, to be able to withstand the stresses, cannot be joined to the remainder of the structure by welding.
It is an object of this invention to overcome the above-described disadvantages of the prior art and to provide a nuclear reactor in which the upper internals including the chimneys shall have the necessary stiffness without being sensitive to thermal inertia matching. It is also an object of this invention to provide an unwelded assembly including the chimneys and their associated structure which shall maintain its integrity in the face of the violent wide temperature fluctuations to which it is subject in use. It is a further object of this invention to suppress the effects of the temperature changes in the coolant during emergencies and particularly during scram.